I doubt that it's a binary. Sure, there's a binary that could be used to describe the overall state of the brain, but my assumption is that it continues developing until death, just the maintenance load increases over time until it eventually overtakes the brain's ability to adapt and it looks like it's just declining, but consider what's actually going on for say a stroke victim to recover functionality. The brain is rewiring itself to make up for parts of it that ceased being functional brain matter.

I'd also consider forming new memories to be a kind of development, since they are encoded using neurons, connections, and whatever else goes into making our brains work.